Final answer:
The purpose of China's Great Green Wall is 3. to control desertification.
Step-by-step explanation:
The purpose of China's Great Green Wall is 3. to control desertification. The Great Green Wall is a reforestation project aimed at combating desertification in the northern and western parts of China.
By intentionally increasing vegetation and planting trees, the project helps prevent soil erosion, create a barrier against wind and sandstorms, and ultimately combat desertification.